Auxiliary power unit adaptive cooldown cycle system and method

ABSTRACT

A system and method for adaptively controlling a cooldown cycle of an auxiliary power unit (APU) that is operating and rotating at a rotational speed includes reducing the rotational speed of the APU to a predetermined cooldown speed magnitude that ensures combustor inlet temperature has reached a predetermined temperature value, determining, based on one or more of operational parameters of the APU, when a lean blowout of the APU is either imminent or has occurred, and when a lean blowout is imminent or has occurred, varying one or more parameters associated with the shutdown/cooldown cycle.

TECHNICAL FIELD

The present invention generally relates to auxiliary power units, andmore particularly relates to an adaptive cooldown cycle system andmethod for auxiliary power units.

BACKGROUND

In many aircraft, main propulsion engines not only provide propulsionfor the aircraft, but may also be used to drive various other rotatingcomponents such as, for example, generators, compressors, and pumps, tothereby supply electrical, pneumatic and/or hydraulic power. However,when an aircraft is on the ground, its main engines may not beoperating. Moreover, in some instances the main propulsion engines maynot be capable of supplying the power needed for propulsion as well asthe power to drive these other rotating components. Thus, many aircraftinclude an auxiliary power unit (APU) to supplement the main propulsionengines in providing electrical, pneumatic, and/or hydraulic power. AnAPU may also be used to start the propulsion engines.

An APU is typically a gas turbine engine that includes a combustionsection, a power turbine section, and a compressor section. Duringoperation of the APU, the compressor section draws in and compressesambient air and supplies the air to the combustion section and tovarious pneumatic loads. Fuel is injected into the compressed air withinthe combustion section to produce the high-energy combusted air to thepower turbine section. The power turbine section rotates to drive agenerator for supplying electrical power, via a main shaft, and to driveone or more compressors (e.g., a power compressor and a load compressor)in the compressor section.

When the APU is no longer needed, it is shutdown. During a normal APUshutdown, a cooldown cycle is performed. The cooldown cycle lowerscombustor and atomizer skin temperatures to prevent coking that may becaused by wet fuel on hot metal. As is generally known, coking typicallymanifests itself in poor fuel atomization or combustion. In many cases,upon completing the cooldown cycle, various circuits and/or functionsmay be tested prior to, or as part of, the full shutdown of the APU.

The cooldown cycle is normally developed and validated on a new engine.The cooldown cycle performance is not monitored and remains the samethroughout APU lifetime. However, as an APU ages it may, in someinstances, experience a lean blowout before the cooldown cycle iscomplete. In such instances, the cooldown cycle cannot be relied upon toensure coking is consistently being prevented. Moreover, the variouscircuits and/or functions would not get tested.

Hence, there is a need for a system and method to monitor theshutdown/cooldown cycle performance and adjust it, as necessary, toensure the cycle reliably and consistently prevents coking as the APUages and/or to ensure various circuits and/or functions are tested aspart of the normal APU shutdown. The present invention addresses atleast this problem.

BRIEF SUMMARY

This summary is provided to describe select concepts in a simplifiedform that are further described in the Detailed Description. Thissummary is not intended to identify key or essential features of theclaimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In one embodiment, an auxiliary power unit (APU) system includes an APUconfigured to rotate at a rotational speed, one or more sensors, and anAPU control unit. The sensors are disposed within the APU, and each isconfigured to sense an operational parameter of the APU and supply asensor signal representative of the operational parameter. The APUcontrol unit is in operable communication with the APU and is coupled toreceive the sensor signals from the one or more sensors. The APU controlunit is further coupled to receive a shutdown signal and is operable, inresponse to the shutdown signal, to: (i) reduce the rotational speed ofthe APU to a predetermined cooldown speed magnitude that ensurescombustor inlet temperature has reached a predetermined temperaturevalue, (ii) determine, based on one or more of the sensor signals, whena lean blowout of the APU has occurred, and (iii) when a lean blowouthas occurred, increasing the predetermined cooldown speed magnitude foruse in a subsequent shutdown of the APU.

In another embodiment, a method of adaptively controlling a cooldowncycle of an auxiliary power unit (APU) that is operating and rotating ata rotational speed includes reducing the rotational speed of the APU toa predetermined cooldown speed magnitude that ensures combustor inlettemperature has reached a predetermined temperature value. Adetermination is made, based on one or more of operational parameters ofthe APU, when a lean blowout of the APU has occurred. When a leanblowout has occurred, the predetermined cooldown speed magnitude isincreased for use in a subsequent shutdown of the APU.

In yet another embodiment, an auxiliary power unit (APU) system includesan APU configured to rotate at a rotational speed, one or more sensors,and an APU control unit. The one or more sensors are disposed within theAPU, and each is configured to sense an operational parameter of the APUand supply a sensor signal representative of the operational parameter.The APU control unit is in operable communication with the APU and iscoupled to receive the sensor signals from the one or more sensors. TheAPU control unit includes a plurality of overspeed trip circuits. Eachoverspeed trip circuit is coupled to receive a speed signalrepresentative of the rotational speed of the APU and is configured tosupply an overspeed trip signal when the speed signal indicates therotational speed of the APU has exceeded a threshold speed to therebycause fuel flow to the APU to cease. The APU control unit is furthercoupled to receive a shutdown signal and is operable, in response to theshutdown signal, to: (i) reduce the rotational speed of the APU at apredetermined rate for a predetermined time period to achieve apredetermined cooldown speed magnitude, (ii) supply a speed signal toone of the overspeed trip circuits that indicates the rotational speedof the APU has exceeded a threshold speed when the rotational speed ofthe APU is reduced to the predetermined cooldown speed magnitude, (iii)determine, based on one or more of the sensor signals, when a leanblowout of the APU has occurred, and (iv) when a lean blowout hasoccurred, increasing the predetermined cooldown speed magnitude for usein a subsequent shutdown of the APU. The APU control unit supplies thespeed signal indicates the rotational speed of the APU has exceeded athreshold speed to a different one of the plurality of overspeed tripcircuits each time it receives the shutdown signal.

In still another embodiment, a method of adaptively controlling acooldown cycle of an auxiliary power unit (APU) that is operating androtating at a rotational speed includes reducing the rotational speed ofthe APU to a predetermined cooldown speed magnitude that ensurescombustor inlet temperature has reached a predetermined temperaturevalue. A determination is made, based on one or more of operationalparameters of the APU, as to whether a lean blowout of the APU isimminent. When a lean blowout is imminent, fuel flow is increased toprevent the lean blowout.

Furthermore, other desirable features and characteristics of theadaptive cooldown cycle system and method will become apparent from thesubsequent detailed description and the appended claims, taken inconjunction with the accompanying drawings and the preceding background.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and wherein:

FIG. 1 depicts a schematic block diagram of one embodiment of anauxiliary power unit (APU) system;

FIG. 2 depicts a flowchart of one embodiment of shutdown control logicthat may be implemented in the APU control unit of FIG. 1 forcontrolling the APU during a shutdown/cooldown cycle;

FIG. 3 graphically depicts one implementation of a shutdown/cooldowncycle of the APU of FIG. 1;

FIG. 4 depicts a flowchart of alternative shutdown control logic thatmay be implemented in the APU control unit of FIG. 1 for controlling theAPU during a shutdown/cooldown cycle; and

FIG. 5 depicts a flowchart of a process that may be implemented in theAPU control unit of FIG. 1 for varying the shutdown/cooldown cycle.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. As used herein, the word “exemplary” means “serving as anexample, instance, or illustration.” Thus, any embodiment describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. All of the embodiments describedherein are exemplary embodiments provided to enable persons skilled inthe art to make or use the invention and not to limit the scope of theinvention which is defined by the claims. Furthermore, there is nointention to be bound by any expressed or implied theory presented inthe preceding technical field, background, brief summary, or thefollowing detailed description.

Referring first to FIG. 1, one embodiment of an auxiliary power unit(APU) system 100 is depicted. The depicted APU system 100 includes anAPU 102 and an APU control unit 104. A typical APU includes at least acompressor section, a combustion section, and a turbine section. Thedepicted APU 102 includes a power compressor 106, a combustor 108, aturbine 110, and a load compressor 112. During operation, the powercompressor 106 draws ambient air into an inlet 114, compresses the air,and supplies the compressed air to the combustor 108. It will beappreciated that the power compressor 106 may be implemented using anyone of numerous types of compressors. For example, the power compressor106 may be a single-stage or multi-stage centrifugal and/or axialcompressor system.

The combustor 108 receives the compressed air from the power compressor106, and also receives a flow of fuel from a non-illustrated fuelsource. The fuel and compressed air are mixed within the combustor 108,and the fuel-air mixture is ignited to produce relatively high-energycombustion gas. The combustor 108 may be implemented as any one ofnumerous types of combustors, including can-type combustors,reverse-flow combustors, through-flow combustors, and slingercombustors.

The relatively high-energy combustion gas that is generated in thecombustor 108 is supplied to the turbine 110. As the high-energycombustion gas expands through the turbine 110, it impinges on theturbine blades, which causes the turbine 110 to rotate. It will beappreciated that the turbine 110 may be implemented using any one ofnumerous types of turbines. The turbine 110 includes a shaft 116 thatdrives the power compressor 106 and the load compressor 112.

The load compressor 112, when driven, draws ambient air into the inlet114 and compresses the air. The compressed air may be supplied tovarious non-illustrated pneumatic loads. Thus, as FIG. 1 also depicts, ableed air conduit 118 is coupled to receive compressed air from the loadcompressor 112 and supplies the compressed air, via a bleed valve 120,to the various pneumatic loads. As with the power compressor 106, theload compressor 112 may be implemented using any one of numerous typesof compressors, including a single-stage or multi-stage centrifugaland/or axial compressor system.

The depicted APU 102 also includes a starter motor 122 and a generator124, both of which are coupled to the shaft 116 via a gearbox and oilpump arrangement 126. The starter motor 122, when included, is used torotate the shaft 116 when the APU 102 is being started. The generator124, if included, may be used to generate and supply electrical power tovarious non-illustrated electrical loads. The gearbox and oil pumparrangement 126 help match the rotational speeds of the starter motor122 and generator 124 to the APU 102, and also supplies oil to variousrotating components in the APU 102.

The APU control unit 104 is in operable communication with the APU 102and and is configured to control the APU 102. In particular, the APUcontrol unit 104 includes one or more processors 128 that, in responseto operator commands and feedback from one or more sensors 130 (e.g.,130-1, 130-2, 130-3 . . . 130-N), are configured to execute start-up,operational, and shutdown control logic. The start-up and operationalcontrol logic executed by the APU control unit 104 may be implementedusing any one of numerous conventional start and operational controllogics and will therefore not be further described. The shutdown controllogic is executed by the APU control unit 104 upon receipt of a shutdownsignal 132. The shutdown control logic that is executed by the APUcontrol unit 104 is heretofore unknown and will thus be described inmore detail.

Before describing the shutdown control logic, it is noted that thenumber and type of the one or more sensors 130 included in the APU 102and that supply feedback to the APU control unit 104 may vary. In thedepicted embodiment, the one or more sensors 130 may include one or morespeed sensors, one or more temperature sensors, and/or one or morepressure sensors, just to name a few. Preferably, the one or moresensors 130 include at least a rotational speed sensor that isconfigured to sense the rotational speed of the APU and supply a sensorsignal representative thereof. In some embodiments, the one or moresensors 130 may also include sensors that are used to directly sense, orindirectly derive, combustor inlet temperature, combustor internaltemperature, atomizer skin temperature, various structure temperatures,ambient temperature, exhaust gas temperature, ambient pressure, andcombustor inlet pressure, just to name a few non-limiting examples.

Turning now to FIG. 2, the shutdown control logic 200 is initiated uponreceipt, by the APU control unit 104, of the APU shutdown signal 132(202). The APU shutdown signal 132 may be supplied automatically ormanually. In the depicted embodiment, however, it is manually suppliedin response to an operator (e.g., pilot) operating an appropriate userinterface (e.g., switch, button, etc.). Regardless of the source of theshutdown signal 132, the shutdown control logic 200 then reduces therotational speed of the APU 102 to a predetermined cooldown speedmagnitude (204). It will be appreciated that the predetermined cooldownspeed magnitude may vary from engine type-to-engine type, or evenbetween engines of the same type. In all cases, however, thepredetermined cooldown speed magnitude is preferably selected to be aspeed that ensures at least that the inlet temperature of the combustor108 has reached a predetermined temperature value to prevent coking.

It will additionally be appreciated that the APU control unit 104 may beconfigured to reduce the rotational speed of the APU 102 to thepredetermined cooldown speed using any one of numerous techniques. Forexample, the APU control unit 104 may reduce the rotational speed of theAPU 102 in one speed reduction stage or in multiple speed reductionstages. The APU 102 rotational speed may be reduced at a constantpredetermined rate or a variable rate, or it may be reduced at apredetermined rate for a predetermined time period.

In one particular embodiment, which is depicted in FIG. 3, the APUcontrol unit 104, upon receipt of the shutdown signal (at t₀), reducesthe rotational speed of the APU 102 to the predetermined cooldown speedin two stages—a post-shutdown stage 302 and a cooldown stage 304. In thepost-shutdown stage 302, the rotational speed of the APU 102 is reducedfrom its operational speed (N_(OP)) to a predetermined post-shutdownspeed value 306 at a first speed reduction rate. Thereafter, in thecooldown stage 304, the rotational speed of the APU 102 is reduced tothe predetermined cooldown speed value 308 at a second speed reductionrate. After the cooldown stage 304, fuel flow to the APU 102 is ceasedand the APU 102 fully shuts down. As FIG. 3 clearly depicts, the firstand second speed reduction rates are unequal. As will be describedfurther below, in some embodiments one or both of the first and secondspeed reduction rates may be varied.

Returning now to FIG. 2, while the APU rotational speed is beingreduced, the APU control unit 104 determines if a lean blowout of theAPU has occurred (206). The APU control unit 104 may determine that alean blowout has occurred using any one of numerous techniques. In thedepicted embodiment, the APU control unit 104 makes this determinationbased on one or more of operational parameters of the APU 102. In oneparticular embodiment, the operational parameter is the rotational speedof the APU 102 since, as is generally known, the rotational speed willdrop off relatively rapidly following a lean blowout. As such, the APUcontrol unit 104 is configured to determine the rate of change of therotational speed of the APU 104, and to determine that a lean blowouthas occurred when the rate of change of the rotational speed of the APU104 is greater than a predetermined rate magnitude. In some embodiments,the determination that a lean blowout has occurred could be made usingsensors that are used to directly sense, or indirectly derive, change incombustor inlet temperature, change in combustor internal temperature,change in exhaust gas temperature, change in combustor pressure, just toname a few non-limiting examples.

Regardless of the specific technique used to determine if a lean blowouthas occurred, when a determination is made that one has occurred, theshutdown control logic 200 increases the predetermined cooldown speedmagnitude for use in subsequent shutdowns of the APU 102 (208), and theshutdown process ends. It will be appreciated that the technique used toincrease the predetermined cooldown speed magnitude may vary. Forexample, the predetermined cooldown speed magnitude 308 may be increasedby reducing the second speed reduction rate or, as depicted in FIG. 3,by reducing the time of the cooldown stage 304 by a predetermined amountof time (Δt) (e.g., from t₂ to t_(2-Δ)), while keeping the second speedreduction rate the same.

As FIG. 2 further depicts, when a determination is made that a leanblowout has not occurred, the shutdown control logic 200 continues withthe remainder of the normal shutdown process. Although this may vary, inthe depicted embodiment this results in the APU control unit 104 testingappropriate circuitry and its functionality (210), and thereaftercausing the APU 102 to be fully shutdown (212).

Again, while the circuitry and functionality may vary, in one particularembodiment, and with quick reference back to FIG. 1, the circuitryincludes at least one overspeed trip circuit 134. As is generally known,an overspeed trip circuit 134 is coupled to receive a speed signalrepresentative of the rotational speed of the APU 102 and is configuredto supply an overspeed trip signal when the speed signal indicates therotational speed of the APU 102 has exceeded a threshold speed. Theoverspeed trip signal in turn causes fuel flow to the APU to cease, andthus causes the APU to fully shutdown. In the depicted embodiment, theAPU control unit 104 includes a plurality of overspeed trip circuits 134(e.g., 134-1, 134-2) for redundancy purposes, and only one of theoverspeed trip circuits 134 is tested during each non-lean-blowout APUshutdown process, in alternate fashion. As such, the APU control unit104 is configured to supply a speed signal that indicates the rotationalspeed of the APU 102 has exceeded the threshold speed to a different oneof the plurality of overspeed trip circuits 134 each time it receivesthe shutdown signal. It will be appreciated that in some embodiments,the APU control unit 104 may include only one overspeed trip circuit 134or more than two overspeed trip circuits 134.

As was noted above, in the embodiment depicted in FIG. 2, andillustrated in FIG. 3, the APU control unit 104 is configured toincrease the predetermined cooldown speed magnitude 308 by reducing thetime of the cooldown stage 304 by a predetermined amount of time. Thispredetermined amount of time (Δt) may, however, be overly conservative.Thus, in some embodiments, as depicted in FIG. 4, the APU control unit104 may execute an alternative shutdown control logic 400 thatimplements additional process steps in addition to those depicted inFIG. 2. Before describing these additional process steps, it is notedthat like reference numerals in FIGS. 2 and 4 reference like processsteps, and descriptions of these like process steps will not berepeated.

Turning now to the additional process steps of the alternative shutdowncontrol logic 400, it is seen that when the APU control unit 104determines that a lean blowout of the APU 102 has occurred, itdetermines what is referred to herein as a lean blowout speed (402). Thelean blowout speed is the rotational speed of the APU 102 when the leanblowout occurred.

As FIG. 4 also depicts, the APU control unit 104 then increases thepredetermined cooldown speed magnitude 308 to a speed magnitude that isgreater than the determined lean blowout speed. The increasedpredetermined cooldown speed magnitude 308 is then used during thesubsequent shutdown(s). This shutdown control logic 400 provides moreaccuracy in determining how much to increase the predetermined cooldownspeed magnitude.

It was additionally noted above, that in some embodiments one or both ofthe first and second speed reduction rates associated with thepost-shutdown stage 302 and the cooldown stage 304, respectively, may bevaried. The general process for implementing this functionality isdepicted in FIG. 5 and will be momentarily described. It should be notedthat this process 500 may be implemented in conjunction with or insteadof the shutdown control logic 200, 400 depicted in FIGS. 2 and 4.

Turning now to the process 500, it too is initiated by the APU controlunit 104 upon receiving the shutdown signal 132 (502). Thereafter, thenormal shutdown/cooldown cycle is initiated (504) and a determination ismade as to whether the shutdown/cooldown cycle can be (or needs to be)adjusted (506). If not, the normal shutdown/cooldown cycle is completed(508). If it is determined that the shutdown/cooldown cycle can beadjusted, then the cycle is adjusted (510). The shutdown/cooldown cyclecan be adjusted by, for example, varying one or both of the first andsecond speed reduction rates.

The technique implemented in the APU control unit 104 to determinewhether the shutdown/cooldown cycle can be adjusted may vary. In oneembodiment, however, the APU control unit 104 uses operationalparameters that are directly measured using one or more of the sensors130, or that are derived from operational parameters measured by the oneor more sensors 130. Some examples of the operational parametersinclude, but are not limited to, combustor temperature, atomizer skintemperature, various structure temperatures, ambient temperature, andexhaust gas temperature. One or more of these temperatures could be usedduring the shutdown/cooldown cycle to ensure the temperatures aresufficiently reduced to prevent coking, before fully shutting down theAPU 102.

In some embodiments, these operational parameters could be used toadjust the speed or time at which the APU 102 operates in thepost-shutdown stage 302 and/or the speed or time at which the APU 102operates in the cooldown stage 304. This would minimize the amount oftime needed to perform the shutdown/cooldown process.

In some other embodiments, APU control unit 104 may be configured, basedon the operational parameters (directly sensed or derived), to determinethat a lean blowout is imminent. In such embodiments, theshutdown/cooldown cycle may be adjusted by increasing fuel flow to thecombustor 108 prevent the lean blowout, while still ensuring sufficientcooldown.

The technique implemented in the APU control unit 104 to determine animminent lean blowout may also vary. In one embodiment, the APU controlunit 104 may be configured to make this determination from the combustorloading characteristic. As is generally known, combustor loading is afunction of ambient pressure, ambient temperature, combustor pressure,combustor temperature, fuel flow, and fuel temperature. As such, the APUcontrol unit 104 may be configured to calculate the current combustorloading factor and compare it against a known combustor load map todetermine if a lean blowout is imminent.

In another embodiment, the APU control unit 104 could monitor combustorpressure for oscillations or other anomalies that might be indicative ofsmall blow outs with auto-ignite occurring. In yet another embodiment,the APU control unit 104 could be configured to monitor combustorpressure during every shutdown. The combustor pressure is then used totrend combustor health, specifically looking signs that is gettingplugged with dirt, debris, corrosion, etc., and compare the trend to theknown combustor loading curve or use a predetermined threshold todetermine when to vary the shutdown/cooldown cycle.

Regardless of the technique used to determine an imminent blowout, ineach embodiment, the APU control unit 104, in controlling theshutdown/cooldown cycle, gives priority to preventing lean blowout asopposed to achieving the lowest temperature to prevent coking.

The system and method described herein provide a means for monitoringthe shutdown/cooldown cycle performance and adjusting it, as necessary,to ensure the cycle reliably and consistently prevents coking as the APUages and/or to ensuring various circuits and/or functions are tested aspart of the normal APU shutdown.

In one embodiment, an auxiliary power unit (APU) system includes an APUconfigured to rotate at a rotational speed, one or more sensors, and anAPU control unit. The sensors are disposed within the APU, and each isconfigured to sense an operational parameter of the APU and supply asensor signal representative of the operational parameter. The APUcontrol unit is in operable communication with the APU and is coupled toreceive the sensor signals from the one or more sensors. The APU controlunit is further coupled to receive a shutdown signal and is operable, inresponse to the shutdown signal, to: (i) reduce the rotational speed ofthe APU to a predetermined cooldown speed magnitude that ensurescombustor inlet temperature has reached a predetermined temperaturevalue, (ii) determine, based on one or more of the sensor signals, whena lean blowout of the APU has occurred, and (iii) when a lean blowouthas occurred, increasing the predetermined cooldown speed magnitude foruse in a subsequent shutdown of the APU.

These aspects and other embodiments may include one or more of thefollowing features. The APU control unit may also be configured, inresponse to the shutdown signal, to reduce the rotational speed of theAPU at a predetermined rate. The APU control unit may also beconfigured, in response to the shutdown signal, to reduce the rotationalspeed of the APU at the predetermined rate for a predetermined timeperiod. The APU control unit may also be configured, in response to theshutdown signal, to reduce the rotational speed of the APU to apredetermined post-shutdown speed value at a first speed reduction rate,and thereafter reduce the rotational speed of the APU to thepredetermined cooldown speed value at a second speed reduction rate. TheAPU control unit may be further configured to selectively adjust one orboth of the first and second speed reduction rates. The APU control unitmay also include at least one overspeed trip circuit, the at least oneoverspeed trip circuit is coupled to receive a speed signalrepresentative of the rotational speed of the APU and is configured tosupply an overspeed trip signal when the speed signal indicates therotational speed of the APU has exceeded a threshold speed. The APUcontrol unit may be further configured to supply a speed signal thatindicates the rotational speed of the APU has exceeded the thresholdspeed to the at least one overspeed trip circuit when the rotationalspeed of the APU is reduced to the predetermined cooldown speedmagnitude, where the overspeed trip signal causes fuel flow to the APUto cease. The APU control unit may also include a plurality of overspeedtrip circuits, and may be further configured to supply the speed signalthat indicates the rotational speed of the APU has exceeded thethreshold speed to a different one of the plurality of overspeed tripcircuits each time it receives the shutdown signal. The APU control unitmay be further configured to determine a rate of change of therotational speed of the APU, and determine that the lean blowout of theAPU has occurred when the rate of change of the rotational speed of theAPU is greater than a predetermined rate magnitude. The APU control unitmay be further configured, upon occurrence of the lean blowout of theAPU, to determine a lean blowout speed, the lean blowout speed being therotational speed of the APU when the lean blowout occurred, and increasethe predetermined cooldown speed magnitude to greater than the leanblowout speed.

In another embodiment, a method of adaptively controlling a cooldowncycle of an auxiliary power unit (APU) that is operating and rotating ata rotational speed includes reducing the rotational speed of the APU toa predetermined cooldown speed magnitude that ensures combustor inlettemperature has reached a predetermined temperature value. Adetermination is made, based on one or more of operational parameters ofthe APU, when a lean blowout of the APU has occurred. When a leanblowout has occurred, the predetermined cooldown speed magnitude isincreased for use in a subsequent shutdown of the APU.

These aspects and other embodiments may include one or more of thefollowing features. Reducing the rotational speed of the APU by reducingthe rotational speed at a predetermined rate. Reducing the rotationalspeed of the APU by reducing the rotational speed of the APU at thepredetermined rate for a predetermined time period. Reducing therotational speed of the APU to a predetermined post-shutdown speed valueat a first speed reduction rate, and thereafter reducing the rotationalspeed of the APU to the predetermined cooldown speed value at a secondspeed reduction rate. The method may further include selectivelyadjusting one or both of the first and second speed reduction rates.When the rotational speed of the APU is reduced to the predeterminedcooldown speed magnitude, at least one overspeed trip circuit may becaused cease fuel flow to the APU. A different one of a plurality ofoverspeed trip circuits may be caused to cease fuel flow to the APU eachtime the cooldown cycle of the APU is initiated.

In yet another embodiment, an auxiliary power unit (APU) system includesan APU configured to rotate at a rotational speed, one or more sensors,and an APU control unit. The one or more sensors are disposed within theAPU, and each is configured to sense an operational parameter of the APUand supply a sensor signal representative of the operational parameter.The APU control unit is in operable communication with the APU and iscoupled to receive the sensor signals from the one or more sensors. TheAPU control unit includes a plurality of overspeed trip circuits. Eachoverspeed trip circuit is coupled to receive a speed signalrepresentative of the rotational speed of the APU and is configured tosupply an overspeed trip signal when the speed signal indicates therotational speed of the APU has exceeded a threshold speed to therebycause fuel flow to the APU to cease. The APU control unit is furthercoupled to receive a shutdown signal and is operable, in response to theshutdown signal, to: (i) reduce the rotational speed of the APU at apredetermined rate for a predetermined time period to achieve apredetermined cooldown speed magnitude, (ii) supply a speed signal toone of the overspeed trip circuits that indicates the rotational speedof the APU has exceeded a threshold speed when the rotational speed ofthe APU is reduced to the predetermined cooldown speed magnitude, (iii)determine, based on one or more of the sensor signals, when a leanblowout of the APU has occurred, and (iv) when a lean blowout hasoccurred, increasing the predetermined cooldown speed magnitude for usein a subsequent shutdown of the APU. The APU control unit supplies thespeed signal indicates the rotational speed of the APU has exceeded athreshold speed to a different one of the plurality of overspeed tripcircuits each time it receives the shutdown signal.

These aspects and other embodiments may include one or more of thefollowing features. The APU control unit may be configured, in responseto the shutdown signal, to reduce the rotational speed of the APU to apredetermined post-shutdown speed value at a first speed reduction rate,and thereafter reduce the rotational speed of the APU to thepredetermined cooldown speed value at a second speed reduction rate,wherein the first and second speed reduction rates are variable. The APUcontrol unit may be further configured to determine a lean blowoutspeed, the lean blowout speed being the rotational speed of the APU whenthe lean blowout occurred, and increase the predetermined cooldown speedmagnitude to greater than the lean blowout speed.

In still another embodiment, a method of adaptively controlling acooldown cycle of an auxiliary power unit (APU) that is operating androtating at a rotational speed includes reducing the rotational speed ofthe APU to a predetermined cooldown speed magnitude that ensurescombustor inlet temperature has reached a predetermined temperaturevalue. A determination is made, based on one or more of operationalparameters of the APU, as to whether a lean blowout of the APU isimminent. When a lean blowout is imminent, fuel flow is increased toprevent the lean blowout.

Those of skill in the art will appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Some ofthe embodiments and implementations are described above in terms offunctional and/or logical block components (or modules) and variousprocessing steps. However, it should be appreciated that such blockcomponents (or modules) may be realized by any number of hardware,software, and/or firmware components configured to perform the specifiedfunctions. To clearly illustrate this interchangeability of hardware andsoftware, various illustrative components, blocks, modules, circuits,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention. For example, anembodiment of a system or a component may employ various integratedcircuit components, e.g., memory elements, digital signal processingelements, logic elements, look-up tables, or the like, which may carryout a variety of functions under the control of one or moremicroprocessors or other control devices. In addition, those skilled inthe art will appreciate that embodiments described herein are merelyexemplary implementations.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC.

Techniques and technologies may be described herein in terms offunctional and/or logical block components, and with reference tosymbolic representations of operations, processing tasks, and functionsthat may be performed by various computing components or devices. Suchoperations, tasks, and functions are sometimes referred to as beingcomputer-executed, computerized, software-implemented, orcomputer-implemented. In practice, one or more processor devices cancarry out the described operations, tasks, and functions by manipulatingelectrical signals representing data bits at memory locations in thesystem memory, as well as other processing of signals. The memorylocations where data bits are maintained are physical locations thathave particular electrical, magnetic, optical, or organic propertiescorresponding to the data bits. It should be appreciated that thevarious block components shown in the figures may be realized by anynumber of hardware, software, and/or firmware components configured toperform the specified functions. For example, an embodiment of a systemor a component may employ various integrated circuit components, e.g.,memory elements, digital signal processing elements, logic elements,look-up tables, or the like, which may carry out a variety of functionsunder the control of one or more microprocessors or other controldevices.

When implemented in software or firmware, various elements of thesystems described herein are essentially the code segments orinstructions that perform the various tasks. The program or codesegments can be stored in a processor-readable medium or transmitted bya computer data signal embodied in a carrier wave over a transmissionmedium or communication path. The “computer-readable medium”,“processor-readable medium”, or “machine-readable medium” may includeany medium that can store or transfer information. Examples of theprocessor-readable medium include an electronic circuit, a semiconductormemory device, a ROM, a flash memory, an erasable ROM (EROM), a floppydiskette, a CD-ROM, an optical disk, a hard disk, a fiber optic medium,a radio frequency (RF) link, or the like. The computer data signal mayinclude any signal that can propagate over a transmission medium such aselectronic network channels, optical fibers, air, electromagnetic paths,or RF links. The code segments may be downloaded via computer networkssuch as the Internet, an intranet, a LAN, or the like.

Some of the functional units described in this specification have beenreferred to as “modules” in order to more particularly emphasize theirimplementation independence. For example, functionality referred toherein as a module may be implemented wholly, or partially, as ahardware circuit comprising custom VLSI circuits or gate arrays,off-the-shelf semiconductors such as logic chips, transistors, or otherdiscrete components. A module may also be implemented in programmablehardware devices such as field programmable gate arrays, programmablearray logic, programmable logic devices, or the like. Modules may alsobe implemented in software for execution by various types of processors.An identified module of executable code may, for instance, comprise oneor more physical or logical modules of computer instructions that may,for instance, be organized as an object, procedure, or function.Nevertheless, the executables of an identified module need not bephysically located together but may comprise disparate instructionsstored in different locations that, when joined logically together,comprise the module and achieve the stated purpose for the module.Indeed, a module of executable code may be a single instruction, or manyinstructions, and may even be distributed over several different codesegments, among different programs, and across several memory devices.Similarly, operational data may be embodied in any suitable form andorganized within any suitable type of data structure. The operationaldata may be collected as a single data set, or may be distributed overdifferent locations including over different storage devices, and mayexist, at least partially, merely as electronic signals on a system ornetwork.

In this document, relational terms such as first and second, and thelike may be used solely to distinguish one entity or action from anotherentity or action without necessarily requiring or implying any actualsuch relationship or order between such entities or actions. Numericalordinals such as “first,” “second,” “third,” etc. simply denotedifferent singles of a plurality and do not imply any order or sequenceunless specifically defined by the claim language. The sequence of thetext in any of the claims does not imply that process steps must beperformed in a temporal or logical order according to such sequenceunless it is specifically defined by the language of the claim. Theprocess steps may be interchanged in any order without departing fromthe scope of the invention as long as such an interchange does notcontradict the claim language and is not logically nonsensical.

Furthermore, depending on the context, words such as “connect” or“coupled to” used in describing a relationship between differentelements do not imply that a direct physical connection must be madebetween these elements. For example, two elements may be connected toeach other physically, electronically, logically, or in any othermanner, through one or more additional elements.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention. It being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. An auxiliary power unit (APU) system, comprising:an APU configured to rotate at a rotational speed; one or more sensorsdisposed within the APU, each of the one or more sensors configured tosense an operational parameter of the APU and supply a sensor signalrepresentative of the operational parameter; and an APU control unit inoperable communication with the APU and coupled to receive the sensorsignals from the one or more sensors, the APU control unit furthercoupled to receive a shutdown signal and configured, in response to theshutdown signal, to: (i) reduce the rotational speed of the APU to apredetermined cooldown speed magnitude that ensures combustor inlettemperature has reached a predetermined temperature value, (ii)determine, based on one or more of the sensor signals, when a leanblowout of the APU has occurred, and (iii) when a lean blowout hasoccurred, increasing the predetermined cooldown speed magnitude for usein a subsequent shutdown of the APU.
 2. The APU system of claim 1,wherein the APU control unit is configured, in response to the shutdownsignal, to reduce the rotational speed of the APU at a predeterminedrate.
 3. The AU system of claim 2, wherein the APU control unit isconfigured, in response to the shutdown signal, to reduce the rotationalspeed of the APU at the predetermined rate for a predetermined timeperiod.
 4. The APU system, of claim 1, wherein the APU control unit isconfigured, in response to the shutdown signal, to: reduce therotational speed of the APU to a predetermined post-shutdown speed valueat a first speed reduction rate, and thereafter reduce the rotationalspeed of the APU to the predetermined cooldown speed magnitude at asecond speed reduction rate.
 5. The APU system of claim 4, wherein theAPU control unit is further configured to selectively adjust one or bothof the first and second speed reduction rates.
 6. The APU system ofclaim 1, wherein: the APU control unit includes at least one overspeedtrip circuit, the at least one overspeed trip circuit is coupled toreceive a speed signal representative of the rotational speed of the APUand is configured to supply an overspeed trip signal when the speedsignal indicates the rotational speed of the APU has exceeded athreshold speed; the APU control unit is further configured to supply aspeed signal that indicates the rotational speed of the APU has exceededthe threshold speed to the at least one overspeed trip circuit when therotational speed of the APU is reduced to the predetermined cooldownspeed magnitude; and the overspeed trip signal causes fuel flow to theAPU to cease.
 7. The APU system of claim 6, wherein: the APU controlunit includes a plurality of overspeed trip circuits; and the APUcontrol unit is further configured to supply the speed signal thatindicates the rotational speed of the APU has exceeded the thresholdspeed to a different one of the plurality of overspeed trip circuitseach time it receives the shutdown signal.
 8. The APU system of claim 1,wherein the APU control unit is further configured to: determine a rateof change of the rotational speed of the APU; and determine that thelean blowout of the APU has occurred when the rate of change of therotational speed of the APU is greater than a predetermined ratemagnitude.
 9. The APU system of claim 1, wherein the APU control unit isfurther configured, upon occurrence of the lean blowout of the APU, to:determine a lean blowout speed, the lean blowout speed being therotational speed of the APU when the lean blowout occurred; and increasethe predetermined cooldown speed magnitude to greater than the leanblowout speed.